The present invention is directed to equalizer circuits, and especially to adaptive equalizer circuits for use in equalizing received signals that have been conveyed over a length of a transmission medium, such as a communication cable. The present invention may be advantageously employed for equalizing a received signal that includes high frequency and low frequency component signals. The present invention is especially useful in equalizing communication signals received over a cable or similar signal conveyance. The exemplary embodiment of the present invention disclosed herein is an equalization apparatus in a receiver in a data communication system that receives signals from a communication cable. The equalization apparatus of the present invention employs signal equalization to compensate for the low pass characteristics of a communication cable. The scope of structure, application and employment of the present invention is not intended to be limited to the exemplary system discussed herein.
Communication systems include numerous communication links, each link involving one or more cables (or other transmission media). Transmission of a communication signal over a cable typically causes distortion of the signal. This distortion generally occurs as frequency dependent attenuation and variations in the signal which are affected by cable length, temperature, interconnection losses and other factors. The signal distortion is often manifested in inter-symbol interference. Data transmitted over a long length of cable at high data rates must be equalized in order to compensate for the loss and phase dispersion imposed by the low pass and other characteristics of the cable and other factors contributing to signal distortion. The extent to which the equalizer is able to match the inverse of the cable loss characteristics determines the extent to which inter-symbol interference induced by the cable distortions can be reduced.
There have been several attempts at providing appropriate signal equalization. Some attempts have sufficed for low speed communication systems having high voltage equipment, on the order of 5 volts and higher. There has been less success in providing an equalizer apparatus for operating at high speeds (for example, 1 gigabit per second or faster) in low voltage systems (for example, less than 3 volts).
Baker (U.S. Pat. No. 5,978,417 for “Adaptive Cable Equalizer”, issued Nov. 2, 1999; and “An Adaptive Cable Equalizer for Serial Digital Video Rates to 400 Mb/s”, IEEE Solid-State Circuits Conference Digest of Technical Papers; pp. 174-175, 1996) disclosed an equalizer apparatus suitable for operation with relatively low data rates, or speeds. Operation of Baker's apparatus at higher speeds (for example, approximately 1 gigabit per second) would not provide acceptable performance. Baker's apparatus provides gain control using current sources. Greater operating speeds and greater gains each contribute to Baker's apparatus requiring higher currents. Higher currents cause greater power dissipation that is manifested principally as increased heat. Higher heat contributes to lowered parts reliability or contributes to using more robust parts to withstand the heat. Thus, greater heat contributes to lowered reliability or to greater cost, and may contribute to both lower reliability and greater cost.
Shakiba (M. H. Shakiba, “A 2.5 Gb/s Adaptive Cable Equalizer”, IEEE Solid-State Circuits Conference Digest of Technical Papers; pp. 396-397, 1999) improves performance over Baker regarding speed and power dissipation. However, Shakiba employs a differential amplifier structure to effect gain control using control voltage variance. The additional differential amplifier structure is connected in cascode with the existing structure. Variance of gain control is also referred to as tuning because one typically varies the gain of the high frequency signal components to offset high frequency losses caused by the low pass characteristic of the cable that delivered the signal being treated. As a consequence of the additional parts required for effecting gain control using control voltage variance, Shakiba requires a higher part count between his supply voltage and ground which means that there are a greater number of voltage drops between supply voltage and ground. This increased voltage drop in Shakiba's apparatus limits voltage headroom and adversely limits voltage swings that can be accommodated by his apparatus.
Cranford (U.S. Pat. No. 5,940,441 for “Integrated Adaptive Cable Equalizer Using a Continuous-Time Filter”, issued Aug. 17, 1999), Babanezhad (U.S. Pat. No. 6,169,764 for “Analog Adaptive Line Equalizer”, issued Jan. 2, 2001) and Gondi (S. Gondi, R. Geiger, J. Liu, J. Bareither, S. Sterrantino and E. Pace, “A 2V Low-Power 125 Mbaud Repeater Architecture for UTP5 Cables”, European Solid-State Circuits Conference, September 2002) all employ feedback to implement their apparatuses [S. Gondi is the inventor of the present invention]. Feedback is known to create limitations with respect to stability. Whenever one employs feedback in a circuit, one must trade stability for speed of operation. Feedback is not conducive to high speed operations but is conducive to good linearity.
In the context of the present invention, linearity refers to characteristics of a circuit depending upon variance of input voltage levels. Some circuits become very nonlinear when large input signals are applied to them. Differential pair circuits are often non-linear. There are two common ways that non-linearity is reduced: (1) feedback and (2) degeneration. Feedback, as mentioned earlier herein, is not conducive to high speed operations. Degeneration refers to the use of introducing passive components (e.g., resistor or capacitor) at the source (or emitter) of a transistor. The present invention preferably employs degeneration to reduce non-linearity.
There is a need for an apparatus and method for equalizing input signals including high and low frequency signal components that may be advantageously employed in high speed, low voltage operations, with acceptable linearity.
There is a need for an apparatus and method for equalizing input signals including high and low frequency signal components that may be advantageously employed in high speed, low voltage operations and has ample voltage headroom to permit flexibility in operation and accommodate voltage swings in the input signals, with acceptable linearity.